In the semiconductor, electronics and life science industries viscous fluids are frequently dispensed. In the semiconductor industry the demands to reduce the size, weight and power of devices often require minute drops of viscous material to be applied. Non-contact dispensing is preferred because a bare silicon die is extremely fragile and it is deleterious for anything to contact the surface of a semiconductor chip. The manufacturing of printed circuit boards (PCBs) often requires small chips to be attached to a circuit board using viscous adhesives to improve the strength and reliability of the bonds. Many adhesives used in PCB manufacturing are premixed, 2-part epoxies which change viscosity with time and the viscosity change causes problems with dispensing equipment. There is thus a need for a dispensing method and device that is insensitive to viscosity change.
In the life science industry clinical testing to screen for diseases, medical conditions and pharmaceutical development has become increasingly sophisticated. Only minute quantities of reagents, proteins or compounds are required for experimentation. Minute drop volumes of low viscosity fluids in the micro liter and nanoliter range are typical. Non-contact delivery of the fluid is preferred in these applications because the minute drop of viscous fluid is frequently dispensed deep into a micro-well. The dispensing must be very precise and repeatable or erroneous results might be produced. Often fluids in the life science industry are in short supply and very expensive. There is thus a need for a jetting device that minimizes the loss of this precious fluid inside the viscous jet. Further, biological materials are very sensitive to contamination and cross-contamination yet existing devices are complex, difficult to maintain clean and can contribute to contamination or cross-contamination. There is also a need for a jetting device having components in contact with the fluid that are either disposable or easily cleaned.
Viscous jetting technology is similar to the ink jet technology used in computer printers. In both cases a small drop of fluid is jetted from a nozzle and “flies” to the substrate in a non-contact manner. Jetting as used herein refers to non-contact dispensing as compared to contact dispensing. Contact dispensing is the process where a fluid drop on the end of a dispensing tip comes in contact with the target substrate and “wets” or clings to the surface and remains on the surface as the dispensing tip pulls away. In the case of ink jet technology, inks with a viscosity very near water (<10 millipascal-seconds—mPas) are jetted. In the case of viscous jet technologies, fluids with high viscosities (>50 mPas) can be jetted. Examples of viscous fluids include adhesives, fluxes, oils, lubricants, conformal coatings, paints, slurries, UV inks, solvents, reagents, proteins, and enzymes. As used herein, high viscosity fluids have a viscosity >50 mPas.
To produce a free flying jetted drop, a rapid high-pressure condition must exist that transfers enough momentum to force fluid through a nozzle with the appropriate exit velocity for the fluid to break into a free flying drop. There here is a specific range of exit velocities that will produce high-quality jetting. The ability to precisely control the rapid, high-pressure condition and eliminate momentum transfer losses and thus control the exit velocity of the fluid from the dispensing orifice would be beneficial for producing repeatable, high-quality jetted drops of viscous fluid.
Creating a rapid high-pressure condition can be achieved in several ways. For example, one such viscous jetting technique uses a reciprocating solenoid valve. A fluid reservoir is pressurized to a predetermined level, a fluid stream from a reservoir flows through a passage extending through a valve assembly having a valve seat disposed near an outlet end in communication with a dispensing orifice. The fluid stream is broken into small drops by a reciprocating valve stem that is cycled by energizing an air solenoid. The action of closing the valve and the impact of the valve stem hitting the valve seat produces a rapid, high-pressure condition that ejects a drop so it “flies” to the substrate in a non-contact manner. The size of the jetted drop is determined by the flow rate of the fluid stream, the size of the valve stem and seat, and the cycle time of the reciprocating valve stem. As a result of deploying a moving mechanical element within the fluid stream a dynamic fluid seal is required. This dynamic seal is subject to wear with time and generates particles due to the sliding action of the seal. Often it is not just the seal that wears, but also the sliding part can wear and generate particles. These particles are cast off into the fluid as contaminants and can cause serious problems for the highly pure fluids as used in life science applications. The wetted moving parts can be hard to clean by flushing, and can require disassembly and costly, time-consuming repair or replacement. The use of fluid seals also requires the moving parts to be constructed of a wear resistant material or have a wear resistant coating which adds cost to the overall design. The internal geometry to support the reciprocating valve stem makes it a challenge to clean and disassembly is always required to ensure cleanliness especially around the dynamic seal area. There is thus a need for a jetting device and technique that does not require dynamic fluid seals.
Another example of creating a rapid high-pressure condition can be seen in a viscous jetting technique which uses a resilient diaphragm impacted by an external element. Fluid from a pressurized reservoir flows into a jetting chamber which is comprised on one side by a resilient diaphragm. An impact means strikes the diaphragm covering a chamber causing a rapid change in chamber volume which generates a rapid, high-pressure condition which ejects a drop of fluid. This viscous jetting technique cannot always adequately meet the drop-to-drop volume accuracy and repeatability requirements. The pressure in the fluid reservoir is rapidly vented to stop the flow of fluid into the jetting chamber. However, often the speed of the pressure drop in the reservoir is slow and a small amount of fluid will escape from the orifice and remain attached to the external surface of the orifice. The fluid attached to the outside of the orifice can affect the volume of the subsequent jetted drop leading to drop-to-drop inaccuracies or it could cause the drop to cling to the orifice. There is thus a need for a viscous jetting technique that minimizes the possibility of an unwanted fluid left on the outside of the orifice after jetting a drop.
Another example of creating a rapid high-pressure condition is a viscous jetting technique that includes a fluid conduit having a flexible tube with a first end connecting to a fluid reservoir and the second end having a dispensing orifice. A displacer connected to a piezoelectric element partially compresses the tube near the outlet end, displaces a volume of tube which creates a rapid high-pressure condition, and jets a drop of fluid. The fluid is refilled in the tube by means of capillary forces. The use of capillary forces to refill the tube limits the speed of refill when using high viscosity fluids. Also, the volume of the jetted drop is not equal to the volume displacement of the tube. The volume displacement forces fluid in two directions: out the orifice and back toward the reservoir. The volume of fluid that flows in each direction depends on the specific flow resistance and compliance of each path. This fluid flow balance between the fluid exiting the orifice and moving backwards toward the reservoir is extremely important in this type of jetting device. Significant variations in drop-to-drop volume can occur if the temperature, pressure or viscosity of the fluid changes the flow balance. The above jetting technique can also be subject to drop-to-drop volume variations as the material of the tube becomes progressively more distorted from use and thus has a variable displacement over time. There is thus a need for a jetting technique that allows fluid in the jetting chamber to only flow out.
It is well known in the industry that a positive metering device, like a syringe pump, an auger pump, or a peristaltic pump can meter an accurate volume of fluid which is substantially independent of the temperature, viscosity and flow characteristics of the fluid and downstream path. While it is possible to use many types of positive metering devices to refill a jet chamber, factors like cost, simplicity, size, serviceability and easy cleaning must be considered. For example, syringe pumps are usually large and bulky and are not easily mounted close to the jetting chamber assembly resulting in a long fluid path between the pump and the solenoid. The increased fluid resistance and compliance can result in a slow response time as the pressure builds slowly inside the long connecting tubing. Also, a one-way valve must be placed in series between the syringe pump and the reservoir which adds cost and complexity to the system. An auger pump can be mounted in close proximity to the jetting chamber reducing the length of the flow path between the pump and the jetting chamber. An auger pump requires a feed screw rotating inside a close fitting chamber. The complexity of the internal mechanism increases cost, is subject to wear, and is hard to clean. An auger feed screw pump would not be desirable when cross-contamination must be avoided. Peristaltic pumps are often used in applications where cross-contamination must be avoided. A typical rotary peristaltic pump employs a plurality of rollers which create squeeze points that moves an occlusion of fluid through a captive tube. The tube material is usually a low-cost, flexible elastomer like silicone which can easily be detached from the pump and disposed, thus eliminating the need to clean the wetted fluid path. However, to accommodate the squeezing motion, the length of the tube is long and the volume of fluid inside the tube is substantial. When using precious fluids the large volume of fluid lost when a tube is replaced is undesirable. There is thus a need for a method of refilling a jetting chamber that meters a precise amount of fluid which is responsive, cost effective and easy to clean.